UNIT-V DEPARTMENT OF INFORMATION TECHNOLOGY::SVECW Page 8
Horizontal/Vertical microinstruction: Each bit of a microinstruction either directly produced a control signal or directly produced one bit of the next address. These schemes require a more complex sequencing logic module.
Packed/Unpacked Microinstruction: The degree of packing relates to the degree of identification between a given control task and specific microinstruction bits.
As the bits become more packed, a given number of bits contains more information. Thus, packing connotes encoding. The terms
horizontal and
vertical relate to the relative width of microinstructions. The terms
hard and soft microprogramming are used to suggest the degree of closeness to the underlying control signals and hardware layout. Hard microprograms are generally fixed and committed to read-only memory. Soft microprograms are more changeable and are suggestive of user microprogramming.
The other pair is direct versus indirect encoding.
Microinstruction Encoding Some degree of encoding is used to reduce control memory width and to simplify the task of microprogramming. The basic technique for encoding is illustrated in Figure a. The microinstruction is organized as a set of fields.
Each field contains a code, which, upon decoding, activates one or more control signals. When the microinstruction is executed, every field is decoded and generates control signals.Thus,
with N fields, N simultaneous actions are specified. Each action results in the activation of one or more control signals Figure 5.8 MicroInstruction Encoding The design of an encoded microinstruction format can now be stated in simple terms
• Organize the format into independent fields. That is, each field depicts a set of actions (pattern of control signals) such that actions from different fields can occur simultaneously.
• Define each field such that the alternative actions that can be specified by the field are mutually exclusive. That is, only one of the actions specified fora given field could occur at a time. Two approaches can betaken to organizing the encoded microinstruction into fields functional and resource. The
functional encoding method identifies functions within the machine and designates fields by function type. For example, if various sources can be used for transferring
data to the accumulator, one field can be designated for this purpose, with each code specifying a different source.
Resource encoding views the machine as consisting of a set of independent resources and devotes one field to each (e.g., IO, memory, ALU). Another aspect of encoding is whether it is director indirect (Figure b.
With indirect encoding, one field is used to determine the interpretation of another field.
